Vasoactive Intestinal Peptide (VIP) is a 28 amino acid peptide previously found in highest concentrations in central and peripheral nervous tissue. Although its physiologic role is not known, VIP is though to be a neuromodulator of intestinal secretion. Using a highly specific radioimmunoassay (RIA) for VIP, we have demonstrated the presence of VIP cross reactive material (VIP-CRM) in a non-innervated tissue; namely, human polymorphonuclear leukocytes (PMNs). VIP-CRM was not measurable in lymphocytes, monocytes, platelets or erythrocytes suggesting this material may be a spcific cell marker for PMNs. Further, our preliminary studies show that, although normal mononuclear cells (lymphocytes and monocytes) do not contain measurable VIP-CRM, mononuclear cells isolated from patients with either acute myelocytic leukemia (AML) or chronic myelocytic leukemia (CML) contained levels of VIP-CRM comparable to those levels observed in PMNs. These initial studies suggest VIP-CRM may be a biochemical cell marker specific for PMNs and their precursors. The purposes of the proposed research are: (1) to evaluate the clinical use of VIP-CRM as a biochemical marker in AML and CML; (2) to confirm the structural identity of VIP-CRM and; (3) to study the role of VIP or VIP-CRM in the function of PMNs and other peripheral blood cells.